Natural IRSL intensities and apparent luminescence ages of single feldspar grains extracted from partially bleached sediments

Radiation Meawrements, Vol. 23. Nos 213, pp. 555-561, 1994 Copyright 0 1994 Ekvier Science Ltd Printed in Great Britain.All rightsresewed

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NATURAL IRSL INTENSITIES AND APPARENT LUMINESCENCE AGES OF SINGLE FELDSPAR GRAINS EXTRACTED FROM PARTIALLY BLEACHED SEDIMENTS M. LAMOTHE;S. BA~ESCIJ and M. AUCLAIR Laboratoire de datation par luminescence LUX, Dkpartement des Sciences de la Terre, Univcrsiti du Quibec i Montrkal, CP 8888 Succ. “A”, Montrtal, Canada H3C 3P8

Abstract-The single aliquot technique has been applied to single grains of K-feldspar extracted from a well-dated late-glacial marine sediment sample for which standard luminescence dating yielded ages that were in excess of the expected age. Natural infrared-stimulated luminescence (IRSL) intensities as well as single grain palaeodoses show a wide range of values. Most of the bright grains yield equivalent doses largely in excess of the expected palaeodose, considering the depositional age of the sample. The luminescence emitted by the brightest grains would dominate the signal emitted from aliquots containing many grains. This explains the overestimation of ages obtained using standard luminescence techniques for the sample investigated. Palaeodoses obtained from grains that exhibit low IRSL intensities are close to the expected dose. However, the age derived from these grains is lower than the expected age. Anomalous fading is believed to be the main cause for the age underestimation. This study is the first demonstration of the feasibility of dating a sedimentary event using the luminescence of a single mineral grain.

1994), the dose measured from optically sensitive traps can also be overestimated owing to poor bleaching at the time of deposition. In such a case, the question then arises of whether the grains selected for dating were all partially bleached before deposition or some have been bleached and others have not been. This paper presents the results of our investigations on the luminescence of single grains of feldspar using infrared-stimulation. The distribution of natural luminescence levels as well as the assessment of the palaeodose of individual feldspar minerals extracted from a late-glacial sand sample (- 10 ka), are taken as evidence that the grain population in this partially bleached sediment is composed of a mixture of unbleached, poorly bleached, and well-bleached grains. The results presented herein have been obtained using the single aliquot additive dose technique developed recently by Duller (1991, 1992). The potential of using the TL of single grains for dating was suggested earlier by Southgate (1985).

1. INTRODUCTION

THERMOLUMI NEKZENCE(TL) and optical dating provide a new and challenging approach to the measure of the time elapsed since deposition of Late Quaternary sediments (Aitken, 1985). However, in the case of partially bleached sediments, the assessment of the original TL level (the so-called residual) is problematic and prone to large errors (Berger, 1988). Methods have been suggested for correctly assessing only the palaeodose acquired after deposition, the most successful being apparently the R-Gamma method of Wintle and Huntley (1980). Even though stratigraphically consistent results have been reported in the literature using this method (for review, see Berger, 1988), this should not be seen as a clear demonstration that the true residual level was necessarily the one deduced from this method. Optically stimulated luminescence (OSL) (Huntley et al., 1985) has therefore emerged as a major improvement for the measure of electrons from optically sensitive traps. Specifically, the infrared-stimulated luminescence (IRSL) of feldspar is now applied to the dating of Quatemary sediments through its virtue of apparently recording the amount of radiation feldspar minerals have received since they were last exposed to sunlight (HPtt and Jack, 1989; Huntley et al., 1985). However, it can be demonstrated that in the case of partially bleached sediments (Balescu and Lamothe, *Author for whom correspondence should be addressed.

2. GEOLOGY

AND PRESUMED AGE OF THE SEDIMENT INVESTIGATED

The sample selected for this study comes from the Saint-Nicolas sand pit located -20 km west of QuCbec City. There, late-glacial marine sand (Fig. 1; Table 1) overlies glacio-marine clay and is in turn overlain by rhythmically banded silt and sand of an estuarine phase of the Champlain Sea. The latter

555

M. LAMOTHE

556

et al.

FIG. 1. Location of the Saint-Nicolas site. The occurrence of interstadial sediments west of Pointe Saint-Nicolas is also shown.

was a short-lived inland marine body of water that invaded giacio-isostaticaliy depressed terrains in Southern Quebec at the end of the last glaciation. The sedimentological and geomorphological context for the site investigated is such that deposition for the sand unit might have occurred under at least 10m of water. Molluscs in the sand have been dated at m 10 ka by radiocarbon (Gadd et al., 1972). A date of 9.7 ka (GSC-1796) obtained on Eihptio freshwater shells found in the close vicinity of the Saint-Nicolas site @Salle, 1989) constrains the age of deposition for the selected sediment sample between 9.5 and 10 ka. The marine sand has been dated by Godfrey-Smith (1991) at 17 and 23 ka, using OSL from quartz and feldspar coarse-grain extracts. It should be noted that these dates were obtained using preheat procedures (e.g. 230°C for 1 s) that would not now be considered as standard. These determinations should be then considered as minimum age estimates. The feldspar coarse-grain extract of an equivalent sample has been dated at -70 ka by Balescu and Lamothe (1994) using both TL total bleach and IRSL additive dose methods (Table I). For both studies, the equivalent doses were found to be in excess of the expected dose by at least 100%.

3. SAMPLE PREPARATION AND LUMINESCENCE MEASUREMENTS Large samples of the sediment analysed by Balescu and Lamothe (1994) have been collected using black plastic tubes, to avoid undue exposure to sunlight at the time of sampling. In the laboratory, the samples were sieved and the grain size fraction for 500 to IOOO~m was retrieved. Such a large grain size was selected so that microprobe analysis could be carried out on the individual grains. Densimetric isolation of feldspar was achieved using solutions of sodium polytungstate to extract those grains with a specific gravity less than 2.58, following the method of Mejdahl (1983). The external m 10 pm surface layer of the grains was etched with dilute HF (10% for 40 min) to reduce the alpha dose rate contribution to a minimum. Single grains of feldspar were mounted on aluminium discs, the grains being attached to the disc with silicone spray. Laboratory irradiations were performed using the McGill University ‘?o facility. Luminescence of the grains was observed using a Coming 7-59-Schott BG39 blue transmitting filter combination, which yields minor green contribution and absorbs most of the unstable ultraviolet component

Table I. Radiocarbon and luminescence dates for the Saint-Nicolas site Dates 10 +0.15 ka BP 9.9;* 0.15 ka BP 17*3 ka 23 f 3 ka 65 f 7 ka 73 f I1 ka

Material

Method

shells Mytiib shells Quartz, feldspar Feldspar Feldspar Feldspar

14c 14C OSL OSL OSL TL

Hemithvris

Remarks GSC-1451 GSC-1508 Kr laser, no preheat Green light, 1 s at 230°C preheat IRSL, 8 h at 160°C preheat Total bleach, 8 h at 160°C preheat

Reference Gadd et al. (1972) Gadd et al. (1972) Godfrey-Smith (1991) Godfrey-Smith (1991) Balescu and Lamothe (1994) Balescu and Lamothe (1994)

IRSL AGES OF SINGLE FELDSPAR (Balescu and Lamothe, 1992). Illumination times were mostly of the order of 1 s, the depletion of the signal being less than 1% for each measurement. The stimulation was achieved with an IRSL unit located on the Daybreak 1100 TL oven constructed with 30 infrared-emitting diodes (Ga Al As, Optek OP296B), running at 30 mA with a peak emission of 880 nm. The power at the sample was 35 mW cmw2. As a single mineral grain covers only a small surface of the sample-holder, the background IRSL signal from a blank aluminium disc was measured. This signal was not significantly above background. The data collection, reduction, and processing were done with the automated Daybreak 1100 system and TLAPPLIC software, at the LUX laboratory (Universitt du Quebec B Montrkal).

part of the IRSL signal from the feldspar minerals. However, this preheat also has the adverse effect of eroding part of the stable IRSL upon each preheat. To correct for this, the reduction of the natural IRSL of other aliquots that were not irradiated is monitored after each preheat cycle and a factor of reduction is applied to correct the measured IRSL signal, this procedure being identical to that suggested by Duller (199 1, 1992). The procedure applied herein is the so-called luminescence correction method. A suite of five growth curves from the selected 15 grains is shown in Fig. 3. The equivalent doses have been calculated according to the Poljakov and Hiitt (1990) exponential fit. Figure 4 shows the distribution of the apparent IRSL ages for these grains. 5. DOSE RATE MEASUREMENTS

4. INFRARED-STIMULATED LUMINESCENCE MEASUREMENTS Natural IRSL intensities have been measured for 120 individual feldspar coarse grains extracted from the Saint-Nicolas sample. The distribution of the natural IRSL intensities is shown in Fig. 2. As can be seen from this figure, there is a large variability of IRSL levels for this sample. Intensities of the order of lo6 photons S-I have been measured for the brightest grains. The 15 feldspar grains that were used for ED measurements are from the low-, mediumand high-intensity ranges. Palaeodoses have been determined using the single aliquot additive dose method developed by Duller (1991, 1992). In this method, IRSL intensities of single aliquots are measured after a preheat of 220°C for 10 min after each irradiation of the same aliquot. This preheat is necessary for removal of the unstable

2500000

557

GRAINS

The external dose rate of the grains has been assessed using the dose rate data presented by Balescu and Lamothe (1994; see Table 2). The internal beta dose has been determined using the K content of the individual feldspar grains, measured with the microprobe facilities available at the Department of Geology of McGill University (Monkal). The internal beta doses have been calculated using the data presented by Mejdhal (1983). Each probe measurement samples a surface of -5 pm diameter, so that N 10 spot determinations were carried out for eight of the 15 grains analysed. The internal K content for the remaining seven grains was estimated using the average value, i.e. _ 11.66% K. As can be seen from Table 2, the internal dose rate for these 500-1000 pm grains accounts for 50% of the total dose rate. At this stage of the investigation, the dosage contribution

T

0

10

20

30

40

50

60

70

80

90

100

110

120

Grrin 1

FIG. 2. Distribution of natural IRSL signals from SOO-1OOOpmfeldspar grains extracted from the Saint-Nicolas marine sand unit.

M. LAMOTHE

558

et al.

from internal consideration.

Rb, U, and Th was not taken into

6. DISCUSSION 6.1, Distribution of IRSL intensities and &termination of equivalent doses The natural IRSL intensities for a suite of 120 grains of the Saint-Nicolas sample show a wide variability in luminescence emission (Fig. 2). It is possible that this could be the result of natural variability in the IRSL emission spectra of the different feldspar grains. This property of feldspar has been well documented by Rendell et al. (1993). This variability

could also result from the intrusion

of older poorly bleached or unbleached grains at the time of deposition, particularly for the brightest grains. The assessment of the equivalent dose is here seen as an appropriate way to check these two hypotheses. The equivalent doses measured for the single grains are shown in Table 3. The doses range from 25 to 322 Gy. These show that, in general, the palaeodoses are systematically higher for grains that originally yielded higher IRSL natural levels (Table 3). This is consistent with our suggestion that the variability of the natural IRSL signal is related to the extent of sunlight exposure at the time of deposition or during

Gamma Dow (Gy)

FIG. 3. Growth curves constructed from single grains, using the single aliquot technique of Duller (1991, 1992). The IRSL intensities are normalized to the natural signal. The grains were gamma irradiated and the data fitted using the exponential equation of Poljakov and Hiitt (1990). The grain numbers shown here do not correspond to those numbers shown in Fig. 2.

70

60

n

60

Apparent IRSL ages

Om-dwP

1

2

3

4

6

6

?

6

6

10

11

12

13

14

16

SW

Qntn t FIG. 4. Apparent IRSL ages for the Saint-Nicolas feldspar grains, compared with the expected 9.5 ka age deduced from the radiocarbon dates and the geological context. The apparent age shown for SUM was obtained by summing the luminescence intensities of the IS grains.

IRSL AGES OF SINGLE FELDSPAR

GRAINS

559

Table 2. Dosimetric parameters for the Saint-Nicolas feldspar grains Grain no.

Grain size’ (pm)

: 3 4 5 6 7 8 9 10 11 12 13 14 15

750750 x 800 750 800x800 1000x700 1100x800 1000X600 870 x 600 640X540 900x800 1000x500 900 x 650 1100x850

Internal beta dose1 (Gy ka-‘)

Internal K content7 (%K)

900X600

1000 x 1500

12.57 11.66

2.94 f* 0.16 2.65 0. I6 2.58 f 0.32 2.15kO.14 2.87 +0.15 3.4 *0.17 2.79+0.15 2.71 kO.15 2.09 f 0.14 2.55 kO.15 2.?6+0.15 2.?0+0.15 3.22 + 0.16 2.58 + 0.15 2.73 kO.15

11.46

9.21 11.46 12.57 11.95 12.25 11.66 10.36 12.47 11.88 11.66 11.66

11.66

Total do@ (Gy ka-‘) 4.14 5.01 f 4.68 f 4.22 f 4.92 + 5.39 + 4.86 f 4.81 f 4.28 + 4.60 f 4.86 + 4.79 * 5.20 + 4.68 * 4.80 *

0.20 0.31 0.19 0.20 0.21 0.20 0.20 0.20 0.20 0.20 0.20 0.21 0.20 0.20

Values in italics are estimated. *Grain size after HF treatment. tMeasured by microprobe. SAwming no contribution from Rb, U, and Th. §Calculated using R. Grim’s AGE program. Soil composition: U [neutron activation analysis (NAA)] = 0.9 f 0.09 ppm; Th (NAA) = 2.9 f 0.3 ppm; K @MA) = 2.5 f 0.05%. Water content = 18 f 5%. Cosmic dose =

150&30mGyka-I. sediment transport. It is believed that some of the grains might have been transported under traction onto the marine basin floor. This is not surprising considering the presumed 10 m depth of water. These grains yield equivalent doses that are far in excess of the expected equivalent dose for a sediment that was deposited shortly before 9.5 ka, given their dose rate. On the other hand, grains 2-7 have low equivalent doses and might be considered as valid dosimeters for the dating of the last depositional event. 6.2. Distribution of apparent IRSL ages

Apparent IRSL ages have been derived using the equivalent doses determined with the single aliquot technique. The ages are shown in Table 3 and Fig. 4. In this figure, the expected depositional age (9.5-10 ka) is shown beside each IRSL apparent age

determination. It can be seen that the ages measured for most of the grains are much larger than the expected age. Some grains yield apparent IRSL ages between 30 and 70 ka, suggesting they may have been derived from older non-glacial sediments such as those found slightly west of Pointe Saint-Nicolas (Fig. 1). It could also be argued that these grains are derived from far older sediments and would have been partially bleached during transport and deposition. The luminescence correction that was used in this investigation is known to yield equivalent doses that could be underestimated and prone to large errors for palaeodoses higher than 100 Gy (Duller, 1992). Therefore, more investigation is required before a definite origin for these grains can be demonstrated. Only grains 2-7 yield ages that are close to the expected age. For these grains, however, the age is

Table 3. Apparent IRSL ages for the Saint-Nicolas feldspar grains Grain no.

Natural IRSL intensity (count s-l)

Annual dose (mGy ka-‘)

Equivalent dose (GY)

Apparent IRSL age (ka)

1 2 3 4 5 6 1 8 9 10 II 12 13 14 I5 SUM

8351 9823 11,877 17,801 21,362 25,023 34,775 41,901 50,195 52,245 590,017 780,743 834,617 1,093,033 2.266.365 5,838,128

5.01 f 0.20 4.14 * 0.20 4.68 f 0.31 4.22 & 0.19 4.92 f 0.20 5.39 & 0.21 4.86 + 0.20 4.81 f 0.20 4.28 f 0.20 4.60 f 0.20 4.86 & 0.20 4.79 f 0.20 5.20 f 0.21 4.68 + 0.20 4.80 f 0.20 4.79 f 0.20

102 f 11.2 39.4 * 4.3 25.4 + 2.8 25.1 f 2.8 28.7 + 3.2 25.1 f 2.3 25.7 k 2.8 lOOk 11.0 138 k 15.2 118 & 13.0 293 f 32.2 184 f 20.2 211 f 23.2 322 f 35.4 306 f 33.7 246 f 27.1

20.4 f 2.4 8.3 f 1.0 5.4 & 0.7 5.9 f 0.7 5.8 f 0.7 6.3 f 0.5 5.3 f 0.6 20.8 f 2.5 32.3 f 3.9 25.7 + 3.1 60.4 f 7.2 38.4 f 4.6 49.6 f 4.9 68.9 f 8.3 63.8 f 7.7 51.4 f 6.2

M. LAMOTHE

560

systematically younger than the true age by 30%. Their low apparent age could be the result of several factors that should be investigated in future work. Some of these are as follows. (1) Part of the age underestimation might be due to a supralinear component in IRSL growth at low doses. However, it is believed that supralinearity should be of the order of a few grays, if there is any. (2) The palaeodoses were measured using the luminescence correction method. In his thesis, Duller (1992) demonstrated that this correction implies that sensitivity remains constant as the grains are being irradiated

and preheated.

A new correction

is there-

fore proposed by this author that takes into account the change in sensitivity as growth proceeds into non-linearity. However, this effect is important only for doses in excess of w 100 Gy. (3) The grains could suffer from severe anomalous fading (Wintle, 1973; Spooner, 1992). In his original work, Duller (1991) used single aliquots of multiple sand-sized grains of feldspar and a preheat of 220°C for 10min (see also Li, 1991). This proved to be sufficient to remove all of the unstable luminescence emission from the New Zealand feldspar grains. Preheat tests have been carried out on previously bleached and irradiated grains (Fig. 5). These results show that the preheat procedure applied herein is not sufficient to eliminate completely the unstable laboratory induced luminescence signal. It remains to be. seen if this could be an indication of the presence of unidentified long-term fading in the sample. Even

0 0

0

q .

0

0 0

0

0 .

0 l

0 .

0 l

.

o

0

. l

0 .

Od-O

Proheatoycln

(22O*C/tO mln)

FIG. 5. Decay of the IRSL signal from natural and irradiated feldsnar arains (2 h bleach by filtered sunlamp f 60 GY beta dosej, priheating at 220°C fdr 10 min at eachcycle. Thi ratio of the two factors is also shown. This ratio shows that eradication of the unstable IRSL would have taken place after at least five cycles of preheat.

et al.

though it cannot be demonstrated at this stage of research, we believe that fading is the main cause for the observed underestimation. A major drawback of using single aliquots with many grains on each is self-evident if one sums the luminescence from the 15 grains (Fig. 4; Table 3). The apparent sum of the natural and irradiated signals yields an equivalent dose of 246 Gy and a corresponding age of 51 ka. This shows that the signal emitted by multiple grains will be dominated by the brightest grains, which could well be the grains that most likely will not have been bleached at the time of deposition. Indeed, the apparent light sum for the 15 grains yields a palaeodose and apparent IRSL age that is close to the 65 ka IRSL age obtained using the IRSL additive dose method with multiple grain aliquots (Balescu and Lamothe, 1994). It is therefore concluded that the use of multiple aliquots, with multiple grains in each aliquot, using an additive dose method (whether total or partial bleach) would yield ages that are in excess of the true age when dating partially bleached sediments. In this investigation, very large grains were used. Therefore, it remains to be seen if this conclusion could be extended to the dating of finer-grained sediments such as glacially derived lacustrine silts (Berger, 1988). 7. CONCLUSION The distribution of natural IRSL intensities for single grains extracted from a poorly bleached sediment shows a wide range of values that are seen as the result of differential bleaching during transport and/or at the time of deposition. The palaeodoses measured using an additive dose single-grain technique confirm that only some of the grains can be used to date the period elapsed since deposition and burial of the sediment. However, the age thus deduced is lower than the expected age. Among several factors that may have caused this age underestimation, fading is taken as the most probable. Dating a sedimentary event using single grains can therefore be achieved, provided the luminescence and particularly the fading behaviour of each individual grain can be assessed. Preheat parameters as well as internal dose rate are grain specific and have to be carefully monitored. Using large grains of Kfeldspars for luminescence measurements has the advantage of reducing the uncertainty related to the environmental dosimetry, because a large proportion of the total dose rate depends upon the internal K content. Even though this single-grain technique can be seen as a tedious and labour-intensive method, the accuracy of the age thus obtained is seen as a major improvement over the standard luminescence techniques. To the knowledge of the authors, this is tY first demonstration of the feasibility of dating sedimentary event using the luminescence of a single mineral grain.

IRSL AGES OF SINGLE FELDSPAR Acknowledgemenu-This contribution was financially supported by an NSERC (Canada) operating grant (GGPOO37375)to M. Lamothe and postdoctoral fellowships from NSERC and PAFACC (UQAM) to S. Balescu. Internal funds from GEOTERAP and the Departement des Sciences de la Terre (UQAM) are also acknowledged. The authors are grateful to F. Hardy and M. Daigneault, who carried out several IRSL laboratory and microprobe measurements. Thanks are extended to McGill University (Montreal) for the use of their gamma source and microprobe facilities.

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GRAINS

561

dans Ie Quebec meridional. 24th International Geological Cohgress, Field Guide Book.

du Quatemaire

Godfrey-Smith D. I. (1991) Optical dating studies of sediments extracts. Ph.D thesis, Simon Fraser University, Bumaby, Canada. Huntley D. J., Godfrey-Smith D. I. and Thewalt M. L. W. (1985) Optical dating of sediments. Nature 313, 105-107.

Hfitt G. and Jaek I. (1989) Infrared stimulated photoluminescence dating of sediments. Ancient TL 7.48-51. LaSalle P. (1989) Stratigraphy and glacial history of the Quebec City region. Friends of the Pleistocene 52nd Annual Reunion, Field Guide Book, pp. 27-70. Li S-H. (1991) Removal of the thermally unstable signal in ontical datina of K-feldsoar. Ancienr TL 9. 26-29. Mejdahl V. (1983)Feldspar inclusion dating of ceramics and burnt stones. PACT 9, 351-364. Poljakov V. and Hiitt G. (1990) Regression analysis of exponential palaeodose growth curves. Ancienr TL 8, 1-2.

Rendell H. M., Khanlary M. R., Townsend P. D., Calderon T. and Luff B. J. (1993) Thermoluminescence spectra of minerals. Mineral. Msg. 57, 217-222. Southgate G. A. (1985) Thermoluminesccna dating of beach and dune sands: potential of single-grain measurements. Nucl. Tracks IO, 743-747. Spooner N. A. (1992) Optical dating: preliminary results on the anomalous fading of luminescence from feldspars. Quat. Sci. Reu. 11, 139-145. Wintle A. G. (1973) Anomalous fading of thermoluminescence in mineral samples. Nature 245, 143-144. Wintle A. G. and Huntley D. J. (1980) Thermoluminescence dating of ocean sediments. Can. .I. Earth Sci. 17, 348-360.